Spiral Arms of the Milky Way

Curious...

Our sun and other stars in the spiral arm we are in, (Orion's Arm),
alternately pass through other spiral arms and inter arm regions . So my
question is, do regions where stars form in, like the Orion Nebula, move
in the same way as the sun?

If the stars are dynamic in position over time, does this mean Orion's
belt and stars like Theta, at the heart of Orion, will someday be part
of another arm within our galaxy?

Spiral Arms of the Milky Way

Roughly speaking, yes. The Galaxy is known to be rotating and we andOrion are rotating with it in such a way that the Orion Nebula will
appear to be where we see it now for a long time to come.

In the Universe there are motions within motions within motions -- more
localized motions matter in this case. Due to perturbations by other
masses (stars, gas clouds, dark matter (maybe), the local motion of a
given body will likely be less regular than its larger motions
(rotation with the rest of the stars of the Galaxy, movement of the
Galaxy itself within the Local Group, and on up the hierarchy.) The
constellations are ephemeral; Orion has looked the way it does today
for a very long time, but in the distant past a hypothetical observer
on Earth would have seen it differently; indeed, if the observer lived
long enough he would have seen Orion taking shape. This goes for the
future, as well; Orion (and the rest of the sky) will look pretty much
the same for a very long time, but not forever. If our observer
continues his very long life he will see Orion break up.

Possibly the most visible short term (circa 26,000 yrs) change is that
the pole star is inconstant due by precession of the Earth's polar
axis. It has not always been Polaris, and it will not always be
Polaris. Please see "Precession" at
<http://www.seds.org/~spider/spider/ScholarX/coord_ch.html> for a good
explanation of some of this motion. This motion, and the motions of
nearby stars are easily detectable in the course just a few years --
well within a human lifetime (also see "Parallax" at the above URL.)

None of this will require you to adjust your polar alignment in the
course of a night's observing, however...

Spiral Arms of the Milky Way

Perhaps we should but some numbers on the time scales here, because
what's "very long" from a human time perspective will be just a blink
of an eye in a cosmic time perspective.

A few millennia will be enough to notice that some stars are moving,
even without special instruments. That's how the first star was
detected to have a proper motion: the star Arcturus was noticed to be
situated one or two degrees differently relative to the background
stars a few centuries ago, compared to the time of the ancient Greeks.

After tens of millennia, many constellations will have changed their
shapes quite noticeably.

After hundreds of millennia, most of today's constellations will be
recognizable only with difficulty - some not at all.

And after millions of years or more, today's constellations will be
almost completely unrecognizable. Some of the more massive stars
will by then even have evolved significantly - Betelgeuze may have
gone nova and then become a white dwarf, for instance.

Consider a swarm of mosqitoes flying by you. Also, imagine that you
were a creature with an extremely short life time, living for perhaps
only 1/100 second or so. During your very short life you see this
swarm of mosqitoes as more or less stationary - perhaps you even
recognize patterns among their apparent positions, and invent
"constellations" of mosqitoes to be able to recognize them. These
constellations would be there all through your life and a number of
generations afterwards. But after hundreds of generations (i.e. after
a few seconds in real time), the mosquitoes will have moved quite
noticeably, and those "constellations" will have changed, or even
vanished, just to be replaced by new constellations, invented by
future generations.

You can view the sky in a similar way: there's a huge of stars there,
all moving relative to one another and relative to the Sun. Now,
humans live some 100 years instead of 1/100 of a second - but otoh
stars live and move over a much longer time scale. So the effect will
be similar: our constellations are nothing but a snapshot of this
swarm of stars, or, more correctly, of that part of the swarm which
happens to be close enough to our own solar system fo the moment.
These constellations will vanish after a suffucuently long time
(thousands of human generations), and perhaps our descendants in the
distant future will invent other constellations of their own?

Halley, Ptolemy, proper motion

Paul Schlyter wrote:

It's true that Halley cited the position of Arcturus (and of Sirius and
Procyon) in the Almagest as support for the idea of proper motion, but
those positions really weren't accurate enough to constitute evidence.
The positions for both Arcturus and Procyon are wrong by half a degree
of latitude, and this is about average for the Almagest star catalog.

It's more accurate to say that Halley became convinced that proper
motion occurred, and that he looked for and claimed to find evidence of
this in the Almagest star catalog. A more convincing case was built,
by Halley and others, using data from much more recent and accurate
observations, starting with those of Tycho.